Preheat and prep: Heat the oven to 350°F (175°C).
Lightly butter a 9x13-inch baking dish or similar.
Slice the apples: Peel if you prefer (not required), core, and slice into 1/2-inch wedges. Thicker slices hold their shape better.
Mix the sauce: In a bowl, whisk melted butter, brown sugar, cinnamon, nutmeg, vanilla, cornstarch, water or cider, and a pinch of salt until smooth.
Combine: Add apple slices to the baking dish. Pour the sauce over and toss to coat evenly.
Stir in nuts or raisins if using.
Bake: Cover the dish with foil and bake for 20 minutes. Remove the foil, stir gently, and bake uncovered for another 20–25 minutes, until the apples are tender and the sauce thickens and bubbles.
Rest: Let the apples sit for 5–10 minutes. The sauce will thicken a bit more as it cools.
Serve: Enjoy warm.
Great with vanilla ice cream, whipped cream, Greek yogurt, or over pancakes and oatmeal.